Solid state cryocooler

ABSTRACT

A cryocooler is disclosed which includes a reservoir ( 11 ), an electrochemical cell or proton conductive membrane (PCM) compressor ( 12 ) coupled to a source of AC current, and a gas expander in the form of a pulse tube expander module ( 13 ). The compressor ( 12 ) includes a proton conductive membrane ( 17 ) positioned between a pair of electrically conductive electrodes ( 18 ) and ( 19 ). The pulse tube expander module  13  includes a regenerator ( 21 ), a pulse tube ( 22 ), and in inertance tube ( 23 ). The regenerator ( 21 ) has a heat rejection part or aftercooler ( 25 ) and a cooling part or cold heat exchanger ( 26 ). The pulse tube ( 22 ) includes a heat rejection portion or hot heat exchanger ( 27 ).

TECHNICAL FIELD

This invention relates generally to cryocoolers, and more particularly to a solid state cryocooler.

BACKGROUND OF THE INVENTION

Since a cryocooler using a Stirling cycle can obtain cryogenic temperatures by repeatedly compressing and expanding a working gas, it has become widely used in cooling operations, such as for cooling of superconducting elements, refining and separation of gases, infrared ray sensors, or the like.

The operation principle of a Stirling cryocooler, using this Stirling cycle, relates to the rising and falling of a compression piston and a displacer in accordance with a refrigeration cycle.

A Stirling cryocooler typically includes a compressor having a compression piston, a regenerator having a regenerating agent, a displacer forming an expansion chamber and a compression chamber, a cooling part formed between the expansion chamber and the regenerator, and a heat rejection part formed around the compression chamber. A working gas is sealed under high pressure in a hermetically sealed flow passage constituted by these members, and the compression piston, of the compressor, and the displacer are reciprocated with a phase difference therebetween.

In the Stirling cryocooler, the compression piston is displaced by mechanical power, so that the pressure of the working gas in the sealed space is changed. The working gas in the expansion chamber is expanded, to cool, using the displacer moving in synchronization with the periodic change of this pressure. Therefore, a high heat efficiency can usually be achieved.

Another type of cryocoolers is known as a pulse tube cryocooler. Pulse tube cryocooler typically include a compressor to repetitively feed and suction a working gas, a regenerator, coupled to the compressor through a heat rejection part and having a regenerating agent, a pulse tube, coupled to the regenerator through a cooling part, and a buffer tank coupled to this pulse tube through a heat rejection part and an inertance tube.

A working gas such as helium, nitrogen or hydrogen can be sealed under high pressure in a hermetically sealed space of this pulse tube cryocooler. Then, similarly to the foregoing Stirling cryocooler, expansion and compression of the working gas is repeated by the compressor to form a pressure amplitude.

In the pulse tube cryocooler the working gas in the pulse tube oscillates in the flow passage, such that it functions as the displacer in the foregoing Stirling cryocooler example. Accordingly, the working gas can be made to work by controlling the phase of the displacement of the oscillating working gas and the pressure wave. Heat is rejected from the heat rejection parts, and heat is absorbed in the cooling part which becomes a cold head of the cryocooler, such that a cryogenic temperature state is formed. The inertance tube and the buffer serve to control the phases of the displacement of the oscillating working gas relative to the pressure wave created by the compressor.

Here, the displacer installed in the Stirling cryocooler is not necessary, and instead of the displacer, the high pressure gas is oscillated so that the working gas can be compressed and expanded. Therefore, there are no movable parts in the low temperature portion. Thus, since mechanical oscillation does not exist at a cooling head, an equipment structure becomes simple, resulting in high efficiency and reliability.

The output (cryocooler output) in the above pulse tube cryocooler is determined by a difference between an output (hereinafter referred to as an indicated cryocooler output) in proportion to the product of a pressure amplitude and a flow amplitude in the inner area of the pulse tube, and various heat losses generated inside the cryocooler. This is represented by the following relation. (refrigeration output)=(indicated refrigeration output)−(heat loss)

A full explanation of these two types of cryocoolers as well as a detailed explanation of the their respective entropies is shown in U.S. Pat. No. 6,691,520, which is specifically incorporated herein by reference with regard to both the prior art and the present invention.

However, with both these types of cryocoolers the expander is driven by a compressor with flexure or coil spring suspended mechanical pistons driven by electromagnetic motor assemblies. The stressing lifetime and reliability requirements result in tight tolerances, labor intensive assembly procedures, and costly materials. The vibration output requirement also contributes to the design complexity of the cryocooler and necessitates expensive control electronics that mitigate the vibration output through closed-loop control of the input current waveform. All of these measures are costly to implement. Furthermore, even with the progress made to date on vibration control, jitter can still be an issue for sensor designers. In short, generation of the pressure wave through a mechanical piston introduces practical limits with respect to vibration output, reliability, lifetime, and packaging, and the industry is collectively approaching those limits.

Accordingly, it is seen that a need remains for a cryocooler that can be operated without creating vibrations and which is reliable for an extended period of time. It is to the provision of such therefore that the present invention is primarily directed.

SUMMARY OF THE INVENTION

In a preferred form of the invention a cryocooler comprises a gas expander, a gas reservoir in fluid communication with the gas expander, and a gas compressor in fluid communication with and between the reservoir and the gas expander. The gas compressor is an electrochemical cell coupled to a source of electricity. With this construction, a gas is compressed by the operation of the electrochemical cell and the gas is subsequently passed to and from the gas expander whereby a refrigeration is produced.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of the cryocooler of the present invention.

FIG. 2 is a schematic view of the membrane electrode assembly of the cryocooler of FIG. 1.

FIG. 3 is a schematic view of multiple membrane electrode assemblies being coupled in series.

FIG. 4 is a schematic view of pressure conditioning system that may be utilized with the cryocooler of FIG. 1.

FIG. 5 is a chart comparing the specific heat temperature dependence of normal hydrogen to helium over a range of pressures and temperatures.

FIG. 6 is a schematic view of the cryocooler of the present invention in another preferred form.

DETAILED DESCRIPTION

With reference next to the drawings, there is shown a solid state cryocooler 10 in a preferred form of the invention. The cryocooler 10 is a closed system which includes a reservoir 11, an electrochemical cell or proton conductive membrane (PCM) compressor 12 coupled to a source of AC current, and a gas expander in the form of a pulse tube expander module 13.

As shown in FIG. 2, the compressor 12 includes an ion conductive membrane such as a proton conductive membrane 17 positioned between a pair of electrically conductive electrodes 18 and 19, as details of which and the operation of which is described in U.S. Pat. No. 6,489,049 and incorporated herein by reference. The pulse tube expander module 13 includes a regenerator 21, a pulse tube 22, and in inertance tube 23. The regenerator 21 has a heat rejection part or aftercooler 25 and a cooling part or cold heat exchanger 26. The pulse tube 22 includes a heat rejection portion or hot heat exchanger 27.

The expander module 13 is based upon the pulse tube design in which the gross refrigeration capacity is achieved passively through the use of fixed, carefully tuned flow geometry. The solid state PCM compressor 12 generates an oscillating hydrogen pressure wave by energizing a proton conductive membrane 17 with an AC current through the electrodes 18 and 19.

The pressure differential across a proton conductive membrane (PCM) results in a chemical potential across the membrane that generates electricity. The use of the PCM compressor is based on the fact that the system is reversible in that ions can be made to flow against the pressure gradient though the application of an excitation current. The proton conductive membrane 17 and the pair of electrodes 18 and 19 form a compressor 12 that allows free passage of working fluid to and from the proton conductive membrane 17 as illustrated in FIG. 2. Electricity is supplied to force the ion flow against the pressure gradient. Positively charged ions pass through the membrane while electrons travel through the electrodes to and from the power supply. The electrodes include a catalyst to promote the electrochemical reactions occurring at each electrode-proton conductive membrane interface. If the system uses hydrogen, the hydrogen gas on the low-pressure side is oxidized resulting in the creation of protons and electrons. The protons are pulled through the membrane 17 by the chemical potential created by the reduction of the hydrogen ions back into hydrogen gas on the high pressure side. An oscillating flow, as is required to drive a pulse tube expander, is created by the excitation of the proton conductive membrane compressor 12 with an AC current.

The compressor 12 described herein has been evaluated against a representative set of pulse tube requirements assuming an arbitrary 70 degrees Kelvin refrigeration temperature (see Table 1). The baseline parameters are based upon a helium design; the actual flow rate is likely lower because of the higher volumetric heat capacity of hydrogen. Simplifying assumptions are made to obtain an input power estimate. It was assumed that hydrogen is compressed uniformly into the expander volume prior to any flow into the pressure reservoir 11. It was further assumed that the amount of mass delivered is sufficient to achieve a stable pressure ratio of 1.3 under isothermal compression at the prescribed operating frequency. Parameter Value Cryocooler operating point 2.0 W @ 70 K Operating temperature of MEA 300 K Pressure ratio across MEA 1.3 Expander volume  4 cc Reservoir volume 50 cc Mean pressure 3.0 MPa Minimum pressure 2.6 MPa Maximum pressure 3.4 MPa EA material example: ZrP MEA electrolyte thickness 1 μm MEA impedance 0.0123 Ωcm² Frequency example: 60 Hz Peak hydrogen mass flow 1.5 g/sec

Voltage is applied to each MEA cell in accordance with the Nernst equation: V _(open circuit) ={overscore (R)}T/2F(ln(P _(ratio))) where {overscore (R)} is the specific gas constant (8.314 kJ/kg° K), T is the cell operating temperature (K), and F is Faraday's constant (96,487 coulombs). For a pressure ratio of 1.3 and a MEA temperature of 300 K, the open circuit voltage is approximately 3.3 mV. Given an objective pumping efficiency of 80%, the maximum allowable voltage drop due to resistance losses must be limited to 0.66 mV. The total voltage across the compressor 17 is 3.96 mv.

Pressure pulses are supplied in sine waves having a RMS mass flow rate of 1.06 g/sec. The mass flow rate is directly proportional to the current flow through the proton conductive membrane compressor 12 stack as given by: $I = \frac{{nAE}\overset{.}{m}}{{MW}_{H_{2}}}$ where n is the number of electrons involved in the process (2 for molecular hydrogen), A is Avogadro's number (6.02e23), E is the charge on a single electron (1.602e-19 C) and MW is the molecular weight of hydrogen gas. Substituting values gives an average current flow of approximately 102 kAmps. For a current flow of 102 kAmps, the proton conductive membrane compressor 12 impedance must be limited to 6.4e-9 ohms in order for the voltage loss due to internal resistance to remain below 0.66 mv. At a resistance of 0.0123 Wcm², the minimum proton conductive membrane 17 area required to achieve the desired electrical efficiency is 1.9 m². The corresponding current flux is 0.053 Amps/cm².

Though the design closes mathematically, the operating current is unacceptably high for practical application. By connecting proton conductive membrane compressors 12 in series (see FIG. 3), the required current can be sufficiently reduced. The voltage is additive due to the series connection. However, the hydrogen flow and current are in parallel across all the compressors 12. Assuming a stack of 105 compressors 12 yields a pulse voltage of 39.6 Volts (3.96 mV each) and a much more practical pulse current of 10.2 Amps.

The required input power is in a range typical of present day pulse tube cryocoolers. The hydrogen is cycled in a sine wave, so pumping power is only applied for the compression half of each cycle. Using an engineering estimate of 60% for the portion of the compression energy recovered during the expansion phase, the power estimate for this analysis comes out to 70 W. Given the conservative assumptions that support this calculation, this estimate compares favorably with the approximately 50 W one would expect to achieve with current state of the art.

With reference next to FIG. 6, there is shown a cryocooler in another preferred form of the invention. Here the cryocooler is a recuperative cryocooler system, rather than the cryocooler of FIGS. 1-5 which is shown to be a regenerative cryocooler system. The recuperative cryocooler system shown in FIG. 6 is a simple Joule-Thomson cycle system, however, it should be understood that any recuperative or regenerative system which uses a compressor may be included in the present invention.

Here, the cryocooler 40 includes a compressor 41, a liquid reservoir 42, a first gas conduit 43 extending between the compressor 41 and the liquid reservoir 42, a second gas conduit 44 extending between the liquid reservoir 42 and the compressor 41, an expansion valve 45 coupled to the first conduit 43, and a heat exchanger 46 is thermal communication with the first and second conduits to transfer heat therebetween. The compressor 41 is an electrochemical cell of the same construction and operation previously recited in detail with regard to the system of FIGS. 1-5.

The operation of the system is essentially the same as conventional Joule-Thomson cycle system except for the novel use of an electrochemical cell as the compressor. The electrochemical cell operates to compress the working fluid thereby forcing it to pass through the first conduit 43, through the expansion valve 45, into the liquid reservoir 42, and then through the second conduit 44 back to the compressor. Here again, the cryocooler operates without vibration as it does not include the moving parts associated with cryocooler compressors of the prior art.

It is believed that the present invention improves the performance and reliability of cryocoolers by completely eliminating all moving components from the design. This approach is inherently reliable, very low in vibration, lightweight, compact, and structurally robust. Electronics are greatly simplified because the need for active vibration control is eliminated. All of these advantages are provided in a cryocooler with thermodynamic efficiency that is competitive with the much more complicated Oxford class designs.

It thus is seen that a cryocooler is now provided which overcomes problems with cryocoolers utilizing mechanical compressors of the prior art. While this invention has been described in detail with particular references to the preferred embodiments thereof, it should be understood that many modifications, additions and deletions, in addition to those expressly recited, may be made thereto without departure from the spirit and scope of the invention as set forth in the following claims. 

1. A cryocooler comprising: a gas expander; a gas reservoir in fluid communication with said gas expander; and a gas compressor in fluid communication with and between said reservoir and said gas expander, said gas compressor being an electrochemical cell coupled to a source of electricity, whereby a gas is compressed by the operation of the electrochemical cell and the gas is subsequently passed to the gas expander whereby a transfer of heat occurs.
 2. The cryocooler of claim 1 wherein said electrochemical cell comprises an ion conductive material, a first electrode mounted upon one side of said ion conductive material, and a second electrode mounted upon one side of said ion conductive material opposite said first electrode.
 3. The cryocooler of claim 2 wherein said ion conductive material is a proton conductive membrane.
 4. The cryocooler of claim 1 wherein said gas expander is a regenerative type unit.
 5. The cryocooler of claim 4 wherein said regenerative type unit is a pulse tube cooler.
 6. The cryocooler of claim 5 wherein said pulse tube cooler includes a regenerator and a pulse tube in fluid communication with said regenerator.
 7. The cryocooler of claim 6 wherein said pulse tube pulse tube cooler further comprises an inertance tube in fluid communication with said pulse tube.
 8. The cryocooler of claim 5 wherein said electrochemical cell comprises an ion conductive material, a first electrode mounted-upon one side of said ion conductive material, and a second electrode mounted upon one side of said ion conductive material opposite said first electrode.
 9. The cryocooler of claim 7 wherein said ion conductive material is a proton conductive membrane.
 10. The cryocooler of claim 1 wherein said gas expander is a recuperative type system.
 11. The cryocooler of claim 10 wherein said recuperative type system includes a liquid reservoir, a first conduit extending between said compressor and said reservoir, a second conduit extending between said reservoir and said compressor, a heat exchanger positioned to exchange heat between said first conduit and said second conduit, and an expansion valve coupled to said first conduit between said heat exchanger and said reservoir.
 12. The cryocooler of claim 1 wherein said electrochemical cell comprises a ion conductive material, a first electrode mounted upon one side of said ion conductive material, and a second electrode mounted upon one side of said ion conductive material opposite said first electrode.
 13. The cryocooler of claim 12 wherein said ion conductive material is a proton conductive membrane.
 14. A cryocooler comprising: a gas reservoir; an electrochemical cell coupled to a source of electricity and mounted in fluid communication with said gas reservoir; a regenerator mounted in fluid communication with said electrochemical cell; and a pulse tube mounted in fluid communication with said regenerator and said gas reservoir, whereby a gas is compressed by the operation of the electrochemical cell and the gas is subsequently passed to the regenerator and pulse tube whereby a transfer of heat occurs.
 15. The cryocooler of claim 14 wherein said electrochemical cell comprises an ion conductive material, a first electrode mounted upon one side of said ion conductive material, and a second electrode mounted upon one side of said ion conductive material opposite said first electrode.
 16. The cryocooler of claim 15 wherein said ion conductive material is a proton conductive membrane.
 17. The cryocooler of claim 14 further comprises an inertance tube in fluid communication with said pulse tube.
 18. A cryocooler comprising: an electrochemical cell coupled to a source of electricity; a liquid reservoir; a first conduit extending between said electrochemical cell and said liquid reservoir; a second conduit extending between said liquid reservoir and said electrochemical cell; a heat exchanger positioned to exchange heat between said first conduit and said second conduit; and and an expansion valve coupled to said first conduit between said heat exchanger and said reservoir, whereby a gas is compressed by the operation of the electrochemical cell and the gas is subsequently passed through the first conduit, the liquid reservoir, and the second conduit whereby a transfer of heat occurs at the liquid reservoir.
 19. The cryocooler of claim 18 wherein said electrochemical cell comprises an ion conductive material, a first electrode mounted upon one side of said ion conductive material, and a second electrode mounted upon one side of said ion conductive material opposite said first electrode.
 20. The cryocooler of claim 12 wherein said ion conductive material is a proton conductive membrane. 